Conventionally, for thread wound golf balls, well known are those in which a core is formed by winding highly expanded rubber thread around a spherical liquid center and a cover with dimples is coated on the rubber thread layer and, then, press-formed into thread wound golf balls. For the liquid center, a hollow section of the hollow spherical center bag comprising rubber, etc. is filled with a liquid containing water or a specific gravity adjusting agent.
The functions required for golf balls include large flying distance and easy ball control. The thread wound golf balls allow larger backspin and ensure better controllability than two-piece solid golf balls, but have disadvantage of shorter flying distance than two-piece solid golf balls.
In the present market, average golfers who want a longer carry prefer two-piece solid golf balls which carry well, and advanced and professional golfers with small handicaps give more emphasis on controllability than flying distance and generally prefer to use thread wound golf balls (particularly balata-covered thread wound golf balls).
In order to increase controllability of two-piece solid golf balls, development has been made to soften the cover and to give the ball a spin like thread wound golf balls. Development has also been made to increase the flying distance of the thread wound golf balls.
However, because increasing the initial speed of balls by increasing the impact resilience of rubber is restricted by the initial-speed upper limit specified by the golf ball specifications, an increase in the flying distance by increasing the initial speed has its own limit.
Consequently, it is necessary to increase flying distance by any other means than increasing initial speed by increasing rubber impact resilience, such as improving liquid center construction.
Therefore, the following description is made on the techniques which have been proposed to date with respect to the center of thread wound golf balls and those which have been actually adopted.
For the center of the thread wound golf ball, there are liquid centers and solid centers.
Since the solid center is a so-called rubber lump and easy to produce, it is proposed in Japanese Kokai Publication No. 48-4025 that the center diameter which used to be 25-28 mm should be increased considerably to 23-39 mm.
In Japanese Kokai Publication No. 59-129072, it is also proposed that the solid center diameter should be increased to 30-38 mm and the specific gravity be reduced while the material with large specific gravity should be used for the cover, thereby increasing the inertia moment and improving carry.
In Japanese Patent Publication No. 60-168471, it is proposed that the solid center diameter should be designed to be as large as 28-32 mm and the cover should be made harder, thereby increasing the delivery angle, reducing spin, and improving carry.
On the other hand, with respect to the liquid center, various researches have been carried out for a long time, and in the specification of British Patent 180,619 (filed in 1921), a method to fill a liquid paste in a spherical hollow section of a mold, refrigerate to make a pellet, cover it with rubber and press-cure, indicating that at that time a manufacturing method of a liquid center had already been developed. In this British patent specification, there is no description on the center diameter nor on dimples.
In the specification of U.S. Pat. No. 2,249,612 or Japanese Kokai Publication No. 48-4026, techniques to cover a liquid paste without refrigeration are disclosed, but in these specifications, there is no description on center diameter nor dimples.
Japanese Patent Publication No. 60-92782 describes that changing the center diameter which used to be 25.4-26.99 mm to 28.6.+-.0.0076 mm has enabled to reduce spin, and Japanese Kokai Publication No. 60-165966 introduces an embodiment in which the center diameter is changed to 28.5 mm, and Japanese Kokai Publication No. 60-187875 recommends that the center diameter be 25-34 mm, preferably 26-30 mm, but these have no description on the combining technique with dimples, and if ever they have, golf balls have diameters of conventional techniques. Japanese Patent Publication No. 62-112575 describes that the center diameter be 20-35 mm, preferably 28-34 mm.
However, this center is formed with high moisture-content gel spherical substance and repeatedly subject to refrigeration, weathering, and defrosting in the manufacturing process. The center is thereby designed to be free from the cover and enabled to be directly wound with rubber thread, and therefore, it should belong to the category of solid center, rather than liquid center.
Japanese Kokai Publication No. 2-255162 describes that the liquid center should be made softer than the conventional one and a load required to deform the liquid center is designed to be lower than the conventional one, thereby reducing backspin and increasing a carry. It also describes that the center diameter is preferably set to 26-29 mm.
Japanese Kokai Publication No. 54-135037 proposes golf balls which use water glass for the center, which is not solid nor liquid, and describes that it is desirable to cover the water glass core 27-31 mm in diameter with a coating material having a thickness of 2 mm or less.
As described above, various proposals have been made in a large number of applications with respect to the center diameter, and researches related to the center diameter have a history of more than 70 years and it thus seems that everything has been already studied.
However, when the diameter of the liquid center of thread wound golf balls actually put into market is investigated, almost all diameters are included in the range of 25-29 mm, and in practice, this range is estimated to be most suited for the application.
This is attributed to the fact that the liquid center cannot hold hardness by the center itself as the solid center can, because the liquid is wrapped with a rubber bag only in the liquid center.
That is, when the center diameter is large, the consumption of rubber thread to be wound decreases, and as a result, a ball with suitable hardness is unable to be obtained, resulting in slower initial ball speed or preventing the ball from carrying high when it is hit. This would be the reasons why diameters greater than about 29 mm have not been adopted.
Of course, there has been proposed that the liquid center diameter be increased to 39 mm, but in actuality, there has been no actual example and it is an armchair theory. Investigation of commercially available golf balls with a liquid center indicates that the actually employed liquid center diameter is within the range of from 25 to 29 mm.
With respect to dimples, Japanese Kokai Publication No. 60-92782 mentioned above proposes the dimple depth and diameter at the center diameter of about 28.6 mm, but this does not exceed the limit of conventional technique as far as the center is concerned. Japanese Kokai Publication No. 54-4626 specifies the dimple of small thread wound balls with a solid center and describes that the desirable center diameter is 25-29 mm. This Japanese reference discloses a technique for optimizing the relationship between the construction and dimples to improve the flying characteristics of thread wound golf balls with solid-center construction, but small balls have presently scarcely been produced and the technique is related to the solid center and is intended for the industrial field different from that of the present invention which is applicable to the liquid center.